Friday, July 6, 2012

Haipad M7/Herotab C8 - Adding Serial ports internally.

This is a post for information only! If you trash your tablet by repeating this I am not responsible!
I killed my tablet with firmware for the Haipad M7+; basically the NAND flash was no longer capable of booting the Linux kernel from either NAND or from SD. There are techniques to get the NAND disabled temporarily, by shorting out pins on the flash chip, but I couldn't get them to work.
Time to add serial ports and work out how to fix it!

Initially I used single core wire, however, that put too much strain on the solder joints, this is cut down cable from a floppy disk drive connector. The trick here is to keep the bundle together, tin the wires carefully, and use a hot iron quickly to press them into contact with the cpu board pins.
I tinned the wires at 220degC and soldered to the board at 320degC. Test for shorts at each stage, not at the end!
Working with a magnifier and with small pins can be disorientating, so I put tape either side of where I was soldering. I use scotch tape as it doesn't leave a sticky residue.
You can see here that I used 6 wires for this, the red wire is for a 5V line, the other wire is for the GND for the serial ports. They're taped out of the way so that I don't melt them with the soldering iron.
When I cut the ribbon cable I left the outer 2 wires longer deliberately to reach the other parts of the circuit board. Pins 0 and 1 on the CPU/Ram/NAND board are GND.
The red line is taped out of the way until I can locate the 5V point I need to power the GPS that I'm thinking of installing. More tape to keep the ribbon cable against the side of the battery and away from the screw holes that secure the back of the case.
Break off a strip of 6 pins for the connector.5V,TX1,RX1,TX0,RX0,GND from left to right. Double sided tape to hold the connector out of the way and a fold of scotch tape to insulate until I work out whether to pierce the case and hot glue gun into a more permanent position.